Prismatic shards are on the AH for less than 1g. There are no blues in Lich King that vendor for less than 1g. This means that players are taking something for which they can get multiple gold and instead getting less than one.

You might be saying, "But I need shards." Okay, so then why are they on the AH? If an enchanter is burning gold for shards for his own enchants, then they won't be on the AH. Alternatively, if you need shards, buy them on the AH rather than making them.

Similarly, low-level essences aren't particularly valuable anymore. Incidentally, this makes them a great way to level enchanting. Weapons tend to make a lot of essences rather than dusts, so they're also often better to vendor rather than disenchant.

Of course your server might be different. Maybe you're on an RP server that only uses shards as currency.

@Dàchéng: You might benefit from larger bags and more efficient use of existing space, such as minimizing the amount you carry that you don't need. A stack of shards may be worth less than a single blue, partial stacks, definitely.

@Kring: Thus reducing the supply of shards and maybe someday, they won't be worthless.

@Matt: I hold on to the shards I get from disenchanting greens. I'm going to bring them next time I do Sha of Anger, for when he asks "Does that make you angry?" I will say, "Yes."